1. Field of the Invention
The present invention relates to a material for a flexible thick film optical waveguide for optical interconnection and a method of preparing the same, and more particularly, to a compound used to prepare a thick film optical waveguide having a thickness of several tens of μm or greater, a prepolymer, a blend, and a polymer sheet obtained therefrom, and an optical waveguide for optical interconnection.
2. Description of the Related Art
Polymer materials for optical waveguides include a large amount of fluorine in order to reduce optical loss in optical communication bands during the fabrication of waveguides. Polymer materials have an intrinsic absorption area based on vibrations of C—H, O—H, and N—H bonds. Secondary and tertiary harmonic overtones of the vibrations which are within a near infrared region, i.e., optical communication bands, directly cause optical loss. The overtone absorption of C—H bond stretching vibrations in hydrocarbon polymers is drastically reduced by replacing C—H with C—F, C—Cl, and C—H2 because the harmonics of the C—F, C—Cl, and C—H2 bonds are longer than that of the C—H bond and are farther from the communication bands, thereby reducing optical absorption in optical communication bands. In particular, a C—F bond may significantly reduce optical loss in the optical communication bands ranging from 1.1 to 1.7 μm. If a C—H bond is replaced by a C—F bond in a polymer to reduce optical loss, polymer solubility may increase so that an optical waveguide may be deformed by a solvent.
Organic materials used for conventional optical waveguides may be solid or liquid. A solid material is dissolved in a proper solvent for fabrication. Thus, there is a need to remove the solvent during the formation of a film. The solid polymer material is formed of an aromatic moiety, solidified after being synthesized, and has a high molecular weight. Thus, the solid polymer has a high viscosity when dissolved. In spin coating that is generally used in the fabrication of a polymer optical waveguide, the high viscosity may affect spin coating process, and thus the increase in the concentration of a polymer solution is limited. Even though the polymer solution is coated to a high thickness due to the high viscosity, it is difficult to obtain a thick film since the solvent is evaporated during the formation of the film. On the other hand, if a polymer solution with a low concentration is used, spin coating is efficiently performed but it is more difficult to obtain a thick film. In order to obtain a thick polymer film using a polymer solution, casting using a doctor-knife may be used instead of spin coating. However, in general, spin coating is used on a silicon wafer in the preparation of an optical waveguide since surface roughness achieved by spin coating is less than that achieved by the doctor-knife. The surface roughness of the optical waveguide is closely related to propagation loss. Since an optical waveguide film prepared using a polymer solution has a high molecular weight and an aromatic moiety, a thick film optical waveguide has excellent mechanical properties.
On the other hand, a liquid material may be simply used in a process of fabricating an optical waveguide. A conventional liquid type polymer material is formed of an aliphatic moiety having a low molecular weight, i.e., an average molecular weight of 1000 or less. Since the molecular weight is not easily increased by a crosslinking process, it is difficult to maintain the shape of the optical waveguide film, and thus a substrate such as a silicon wafer is required. A material mostly composed of aliphatic moieties has poor elasticity and strength, thereby having poor mechanical properties.
As a liquid material for an optical waveguide, photo crosslinkable organic materials with low molecular weight and aliphatic moiety have been developed. Thus, many researches on the variation of optical properties with their combinations have been conducted. An optical waveguide film is formed by UV-curing right after spin coating the photo crosslinkable material. As the UV-curing process proceeds, molecular fluidity decreases, and thus the increase in the molecular weight of the optical waveguide film is limited. This limitation deteriorates the mechanical characteristics of the film. Since the thickness of a film depends on a viscosity of a liquid material during spin coating, and the increase in the viscosity of a low molecular weight liquid is limited, it is difficult to obtain a thick film and to control the thickness of the film.
In order to secure flexibility of an optical waveguide, an organic polymer may be used as a material for forming the optical waveguide. An aliphatic moiety is more efficiently used rather than an aromatic moiety in consideration this flexibility. A siloxane-based material containing silicon may be used in order to secure flexibility of the film. In this regard, organic-inorganic hybrid materials are mainly obtained by siloxane reaction between silane and diol. If the siloxane reaction is not completely performed, optical loss by hydroxyl moieties may be caused. Organic materials containing a large amount of aliphatic hydrocarbon or organic-inorganic hybrid materials containing silicon have high flexibility, but poor elasticity, so that mechanical properties thereof such as tensile strength, abrasion resistance, fatigue resistance, and bending strength may be deteriorated. In order to improve these poor mechanical properties, there is a need to increase the molecular weight of the polymer film or introduce an aromatic moiety thereinto. However, since a polymerized material and a material including a large amount of aromatic moieties may solidify, they need to be dissolved in a solvent to perform spin coating for fabricating a film. This solubilization cause the drawbacks described above.
A variety of organic materials for optical waveguides have been developed. However, most of the conventional materials are not suitable for forming a flexible thick film optical waveguide due to various drawbacks.